The invention relates to an electronic circuit arrangement and a method for self-commutating control of a brushless, permanently excited, direct current motor, the rotor position being determined without additional sensors.
When an electronically commutated electric motor is to be operated as a self-commutating machine, the momentary rotor position has to be detected. Separate sensors such as encoders, resolvers or Hall ICs can be used for this purpose. However, it is also possible to use a sensor-free system which detects the rotor position by evaluating the voltages induced in the motor phases.
U.S. Pat. No. 5,469,033 A discloses a method by which the arrangement of a plurality of Hall sensors in a brushless direct current electric motor is replaced by an electronic circuit arrangement. Signals which this circuit arrangement generates with the aid of a logic unit replace the functions of the Hall sensor signals.
However, limits are imposed on the exact positioning of the Hall sensors, more especially in the case of smaller drives. The mechanical outlay rises significantly. In order to keep the positioning error margin low and obtain a high degree of efficiency and/or high motor torque, the only option open is often to adjust the sensors at considerable expense during the manufacturing process. The result is a considerable increase in costs, particularly for those motors which are very small but nonetheless expected to meet very high performance standards. For such cases the sensor-free system for rotor position detection is an economical alternative despite the higher outlay in electronics. Also in the case of motors in which the rotor is encapsulated, so that additional mechanical outlay is needed in order to pass the Hall sensor signals through the housing, it can be advantageous to use a sensor-free system which detects the rotor position by evaluating the voltages induced in the motor phases. Sensor-free systems for rotor position detection have become an established feature of lower-powered motors which are not subject to exacting requirements with regard to dynamics, and of motors intended to operate within a limited range of speeds. Examples include hard disk drives, pump drives, fans and forced induction engines. This method is based on the detection and evaluation of the electrical signals induced in the phases. Such signals are also known as counter emf signals or back emf signals, and are referred to from here on simply as induction signals.
European patent application EP 0 840 439 A1 discloses an amplifier and a method for determining an induction signal (back emf signal) in a phase of a direct current motor in order to determine the rotor position. In this instance a comparator circuit is used to compare the induction signal with a reference signal. If the reference signal is exceeded the status of the output signal is altered. In order to eliminate unknown factors arising from component-related response hysteresis, a voltage offset is superimposed on both the induction signal and the reference signal, the voltage offset being applied to the reference signal in response to a control signal only when the induction signal approaches a zero passage.
In the known method, evaluation of the induction signal to determine the rotor position assumes that the induction signal has an ideal curve, but this does not correspond to an induction signal that is actually measurable in a motor phase during normal running. In reality such a signal is overlaid with interference and oscillations, caused for example by the electronics. This occurs in particular when the motor is operated in pulse-width modulation mode and is connected to power semiconductors for the purpose of commutation. The signal is therefore overlaid with interference having a bandwidth of unknown frequency and amplitude, complicating the accurate determination of the induction signal curve and thus of the relative rotor position. This in turn, particularly at higher rotational speeds, has the effect of causing appreciable power restrictions during normal running and further restrictions during speed adjustment and control.